Ionization chamber in particular for a fire detection device

ABSTRACT

An ionization detection chamber, in particular for a fire detecting apparatus, of the type comprising an ionizing source and two electrodes each of which constitutes an ion collecting electrode of a given sign. In order to minimize the variation of the ion collection current when the surrounding pressure increases, the chamber further comprises at least one ion collecting member integral with one of the electrodes, said collecting member extending towards the ionizing source in the radiation beam of the latter.

United States Patent [191 Lecuyer [45 1 Feb. 12, 1974 IONIZATION CHAMBER IN PARTICULAR FOR A FIRE DETECTION DEVICE [75] Inventor: Daniel Marie Pierre Lecuyer, Le

Pecq, France [73] Assignee: Vigifeu,-Aubervillias, France [22] Filed: July 15, 1971,

Appl. No.: 162,963

Related US. Application Data [63] Continuation-impart of Ser. No. 790,235, Jan. 10,

I969, abandoned.

[52] US. Cl. 250/83.6 FT, 340/227 R [51] Int. Cl. G0lt 1/18 [58] Field of Search 250/836 FT; 313/54,- 93; 340/227 R [56] References Cited UNITED STATES PATENTS 2,963,600 12/l960 Meili et al 313/54 X 3/1970 Nagao Abe 250/836 FT X 5/1970 Klein 250/836 FT Primary Examinr.lames W. Lawrence Assistant Examiner-Davis L. Willis Attorney, Agent, or Firm-Wenderoth et a].

[57] 7 ABSTRACT An ionization detection chamber, in particular for a fire detecting apparatus, of the type comprising an ionizing source and two electrodes each of which constitutes an ion collecting electrode of a given sign. In order to minimize the variation of the ion collection current when the surrounding pressure increases, the chamber further comprises at least one ion collecting member integral with one of the electrodes, said collecting member extending towards the ionizing source in the radiation beam of the latter.

' 8 Claims, 7 Drawing Figures PATENTED FEB 1 21974 sum 1 OF 2 FIG.1

FIG.3

IONIZATION CHAMBER IN PARTICULAR FOR A FIRE DETECTION DEVICE This application is a continuation'in-part of my application Ser. No. 790,235, filed Jan. 10, 1969 and now abandoned.

The present invention relates to ionization detection chambers, in particular of the type employed in fire detecting devices.

Generally, detection apparatuses comprise a saturated ionization chamberwhich constitutes a reference chamber and an unsaturated ionization chamber through which the surrounding gas can flow and which constitutes a'measuring or detection chamber. The latter comprises an ionizing source and two electrodes each of which constitutes an ion collecting electrode of given sign. The resulting current is compared to the substantially constant current in the reference chamber and the differential current is a function of the nature or composition of the surrounding gas and constitutes a measuring magnitude of the nature or composition of this gas. Such a device is well known in the art.

In prior devices the detection chamber is defined by a cylindrical wall constituting a first electrode and supporting the ionizing source, and further comprises an axial rod located coaxially within said cylindrical wall and which constitutes the second of said two electrodes.

Now, the current collected and consequently the differential current also depends on the pressure prevailing in the detection chamber when this pressure in creases, the air molecules density increases, the mean free path of the :1 particles emitted by the ionizing source decreases and the dimensions of the ionized area are substantially reduced. The probability for the ions to reach the axial collecting electrode decreases and a large variation in the collected currentand in the differential current, for a given voltage, has been observed in known apparatuses when the pressure increases. The ratio between the current intensities at the atmospheric pressure pa and at a pressure such as pa+5kglcm can be such as 30 or 50 in the known fire detecting devices and it will beunderstood that it makes such devices unreliable in usewithin areas such as missile launching tubes, for example, where such high pressures are involved. This phenomenon consti tutes a serious drawback which limits the use of such devices to applications where the pressure remains substantially equal to the normal atmospheric pressure.

The object of the invention is to provide a ionization detection chamber, in particular for a fire detecting device which is improved in such manner that the variations in the intensity of the collected current, as a function of variations in the pressure within the chamber, are reduced to minimum proportions.

The invention provides a non saturated detection ionization chamber for use in a detection device comprising in series said non saturated detection chamber and a saturated reference chamber, said detection device being intended to operate under variable pressure conditions, said detection chamber comprising a substantially cylindricalconductive wall constitutinga first electrode and defining a detection space,said cylindrical wall having at one end a substantially flat bottom wall and being opened to the surrounding atmosphere at the other end, an ionizing source onsaid cylindrical wall, means for allowing a continuous flow of the surrounding gas through said detection space, a conductive rod constituting a second electrode and extending through said bottom wall and being electrically isolated therefrom, said second electrode extending into said detection space coaxially to the cylindrical electrode and supporting intermediate its ends anion collecting member which extends transversally to said rod towards said ionizing source.

Owing to this feature, under high pressure conditions when the ionized area is reduced, the ions of a sign opposite that of the electrode provided with the ion collecting element are very rapidly collected by said element as soon as they move away from the source, and there is a great probability for the ions of different sign to return to their collecting electrode.

As the probability of recombination of the ions is thus highly reduced, the pressure variations in the chamber bring about only limited variations in the col lected current for a given voltage between the two electrodes.

The invention also provides a detection device comprising in series said non saturated detection chamber and a-saturated reference chamber, said detection device being intended to operate under variable pressure conditions, wherein said detection chamber comprises a substantially cylindrical conductive wall constituting a first electrode and defining a detection space, said cylindrical wall having at one end a substantially flat bottom wall and being opened to the surrounding atmosphere at the other end, an ionizing source on said cylindrical wall, means for allowing a continuous flow of the surrounding gas through'saidl detection space, a conductive rod constituting a second electrode and ex tending through said bottom wall and being electrically isolated therefrom, said second electrode extending into said detection space coaxially to the cylindrical electrode and supporting intermediate its ends an ion collecting member which extends transversally to said detection device provided. with a. detection chamber accordingto the invention;

FIGS. 2-4 are diagrammatic views taken transversally to the longitudinal axis of the chamber and showing the effects of pressure increase on the ionization conditions in the detection chamber;

FIG. 5 shows a set of voltampere characteristics of a detection chamber according to the invention, the parameter of the set of curves being "the pressure prevailing inside the chamber; and

FIGS. 6 and 7 show sets of similar characteristics, to a smaller scaleyfor known detection chambers.

In the embodiment shown in F116. 1, the ionization chamber adapted to constitute the detection chamber A of a fire detecting device comprises a cylindrical wall 1 having a bottom wall la and constituting a first or outer electrode, a second or central electrode 2 constituted by a rod located on the axis of and extending through, the cylindrical electrode 1, the electrode 2 being insulatedfrom the electrode 1, an ionizing source 3 secured to a portion of the innerwall of the cylindrical electrode and emitting a suitable a radiation, and an ion collecting element 4 integral with the central electrode 2 and consisting of a disc which is perpendicular tothe electrode 2 and extends towards the source 3 in the radiation beam thereof.

The disc 4 can be conductive and constructed-for example of nickel plated sheet steel and brought to the potential of the electrode 2. It can also be made of an insulating material and then provide a surface ion collection. The diameter of the disc is slightly less than that of the cylindrical electrode 1 so that it very rapidly receives the ions of a sign opposite that of the central electrode 2. The distance between the edge of the disc and the ionizing source can be a few millimetres but it can be still further reduced under certain conditions of utilization.

In the embodiment shown on FIG. 1, the detection chamber according to the invention is incorporated to a fire detection device comprising an outer housing enclosing the above described detection chamber A and a reference chamber B connected in series. The reference chamber comprises a substantially closed cylindrical member 11 supporting on its inner lateral wall a radioactive source 12 which can be the same as in the detection chamber. The conductive rod 2 common to both chambers extends axially into said reference chamber and is electrically isolated therefrom by means of an insulating support member 13.

It will be appreciated that the bottom wall 1a of the detectionchamber is provided with apertures 1b, 1c one of which 1c is located around the central rod 2, whereas similar apertures 14 are provided in the outer housing 10, adjacent said bottom wall la. At its opposite end, the detection chamber A opens into a collecting space 5 limited by a grid 6 supported by said outer housing 10. It will be appreciated that apertures 1b and 14, and the collecting space 5 constitute means allowing the surrounding gas to flow continuously through detection chamber A.

' The circuit means adapted for applying the suitable voltage to the different electrodes and for amplifying the current variations at the common or junction electrode can be of any known type and will not be shown nor described in more details.

Operation of a detection chamber and device incorporating the novel features of the invention will now be described with reference to FIGS. 1, 2, 3, 4 of the drawings.

At atmospheric pressure pa (FlG. 2 the gas density in the detection chamber is such that the mean free path of the a particles emitted from the radioactive source 3, which measures the ionization range is greater than the dimensions of the chamber itself. The

whole space defined by the outer wall electrode l is thus ionized, but the ionization rate is relatively low. It is thus important that the ion collection takes place in the whole chamber, which is realized respectively for the ions of opposite signs by electrode 1 and by electrode 2 and disc 4. The latter are substantially coextensive, respectively axially and transversally with chamber A thus affording a suitable ion collection.

When the pressure increases and reaches, i.e., pa-l-2kg/cm (FIG. 3) the ionization range of source 3 becomes smaller than the diameter of the chamber and the collection area of disc 4 becomes progressively reduced, this being partially compensated by the fact that the ions are formed at a higher rate and are more quickly collected.

When the pressure reaches high magnitudes such as pa+5kglcm (FlG.4) the ionization range is still reduced and the ions are essentially confined in the area shown on the drawing as limited by a phantom line. However and as described above, due to the provision of disc 4, the ions of one sign are collected with high efficiency, as soon as they are formed whereby the ions of opposite sign can be collected by the cylindrical electrode and the recombination is avoided, as would otherwise occur in the absence of such a collecting disc. Consequently, for a given voltage, the current collected decreases in relatively small proportions when the pressure in the chamber increases.

The set of curves shown in FIG. 5 illustrates the operation of the chamber by indicating the variations in the intensity of the collected current I as a function of the applied voltage U (voltampere characteristic), for different values of the pressure p; the curves pa, p p p p p corresponding respectively to atmospheric pressure pa, pa-Hkg/cm pa+2kg/cm pa+3kg/cm pa+4kg/cm pa+5kg/cm By comparing these curves with those of FIGS. 6 and 7 relating to known apparatuses, the great improvement afforded by the invention can be seen in the reduction in the ratio between intensity at atmospheric pressure pa and the intensity at a pressure of pa-l-Skg/cm for a voltage of, for example, volts. In the case of FIG. 5, this ratio is /60 and therefore in the neighborhood of 2.8 whereas in the case of FlG. 6 or 7 it is not less than 30.

This improved ionization chamber can thus be used in a tire detecting apparatus operating within a range of pressures capable of varying between one and several atmospheres, as it is the case in missile launching tubes or other pressurized enclosures.

A detecting apparatus according to the invention will have the advantage of being very slightly sensitive to the effect of variations in pressure owing to the use of the chamber according to the invention.

Although specific embodiments of the invention have been described, many modifications and changes may be made therein without departing from the scope of the invention.

Having now described my invention what I claim and desire to secure by Letters Patent is:

l. A non saturated detection ionization chamber for use in a detection device comprising in series said non saturated detection chamber and a saturated reference chamber, said detection device being intended to operate under variable pressure conditions, said detection chamber comprising a substantially cylindrical conductive wall constituting a first electrode and defining a detection space, said cylindrical wall having at one end a substantially flat bottom wall and being opened to the surrounding atmosphere at the other end, an ionizing source on said cylindrical wall, means for allowing a continuous flow of the surrounding gas through said detection space, a conductive rod constituting a second electrode and extending through said bottom wall and being electrically isolated therefrom, said second electrode extending into said detection space coaxially to the cylindrical electrode and supporting intermediate its ends an ion collecting member which extends transversally to said rod towards said ionizing source.

2. A chamber as claimed in claim 1, wherein said second electrode extends into said first cylindrical electrode over a distance substantially coextensive to the axial length of said first cylindrical electrode and the ion collecting member is located substantially in the middle portion thereof.

3. A chamber as claimed in claim 1, wherein said ion collecting member is a disc of electrically conductive material integral with or fixed to said second electrode and having a peripheral edge spaced by a few millimetres from said ionizing source.

4. A chamber as claimed in claim 1, wherein said ion collecting member is a disc of electrically insulating material and provides a surface collection, said disc having a peripheral edge spaced by a few millimetres from said ionizing source.

5. A fire detecting device specially for operating in a large range of pressures and of the type comprising in series a non saturated ionization detection chamber anda saturated ionization reference chamber, wherein said detection chamber comprises a substantially cylindrical conductive wall constituting a first electrode and defining a detection space, said cylindrical wall having at one end a substantially flat bottom wall and being opened to the surrounding atmosphere at the other end, an ionizing source on said cylindrical wall, means for allowing a continuous flow of the surrounding gas through said detection space, a conductive rod constituting a second electrode and extending through said bottom wall and being electrically isolated therefrom, said second electrode extending into said detection space coaxially to the cylindrical electrode and supporting intermediate its ends an ion collecting member which extends transversally to said rod towards said ionizing source.

6. A device as claimed in claim 5, wherein said second electrode extends into said first cylindrical electrode over a distance substantially coextensive to the axial length of said first cylindrical electrode and the ion collecting member is located substantially in the middle portion thereof.

7. A device as claimed in claim. 5, wherein said ion collecting member is a disc of electrically conductive material integral with or fixed to said second electrode and having a peripheral edge spaced by a few millimetres from said ionizing source.

8. A device as claimed in claim. 5, wherein said ion collecting member is a disc of electrically insulating material and provides a surface collection, said disc having a peripheral edge spaced by a fewmillimetres from said ionizing source. 

1. A non saturated detection ionization chamber for use in a detection device comprising in series said non saturated detection chamber and a saturated reference chamber, said detection device being intended to operate under variable pressure conditions, said detection chamber comprising a substantially cylindrical conductive wall constituting a first electrode and defining a detection space, said cylindrical wall having at one end a substantially flat bottom wall and being opened to the surrounding atmosphere at the other end, an ionizing source on said cylindrical wall, means for allowing a continuous flow of the surrounding gas through said detection space, a conductive rod constituting a second electrode and extending through said bottom wall and being electrically isolated therefrom, said second electrode extending into said detection space coaxially to the cylindrical electrode and supporting intermediate its ends an ion collecting member which extends transversally to said rod towards said ionizing source.
 2. A chamber as claimed in claim 1, wherein said second electrode extends into said first cylindrical electrode over a distance substantially coextensive to the axial length of said first cylindrical electrode and the ion collecting member is located substantially in the middle portion thereof.
 3. A chamber as claimed in claim 1, wherein said ion collecting member is a disc of electrically conductive material integral with or fixed to said second electrode and having a peripheral edge spaced by a few millimetres from said ionizing source.
 4. A chamber as claimed in claim 1, wherein said ion collecting member is a disc of electrically insulating material and provides a surface collection, said disc having a peripheral edge spaced by a few millimetres from said ionizing source.
 5. A fire detecting device specially for operating in a large range of pressures and of the type comprising in series a non saturated ionization detection chamber and a saturated ionization reference chamber, wherein said detection chamber comprises a substantially cylindrical conductive wall constituting a first electrode and defining a detection space, said cylindrical wall having at one end a substantially flat bottoM wall and being opened to the surrounding atmosphere at the other end, an ionizing source on said cylindrical wall, means for allowing a continuous flow of the surrounding gas through said detection space, a conductive rod constituting a second electrode and extending through said bottom wall and being electrically isolated therefrom, said second electrode extending into said detection space coaxially to the cylindrical electrode and supporting intermediate its ends an ion collecting member which extends transversally to said rod towards said ionizing source.
 6. A device as claimed in claim 5, wherein said second electrode extends into said first cylindrical electrode over a distance substantially coextensive to the axial length of said first cylindrical electrode and the ion collecting member is located substantially in the middle portion thereof.
 7. A device as claimed in claim 5, wherein said ion collecting member is a disc of electrically conductive material integral with or fixed to said second electrode and having a peripheral edge spaced by a few millimetres from said ionizing source.
 8. A device as claimed in claim 5, wherein said ion collecting member is a disc of electrically insulating material and provides a surface collection, said disc having a peripheral edge spaced by a few millimetres from said ionizing source. 